The present invention is directed to a phase control circuit for adjusting the power delivered to an AC load. More specifically,.the present invention is directed to a phase control circuit having independent control of each half cycle, which is particularly useful in controlling a gas discharge lamp.
Gas discharge lamps are used in a variety of applications. For example, mercury vapor lamps are used for ultraviolet (UV) curing of ink in printing presses, for curing furniture varnish, in germicide equipment for killing germs in food and its packaging, and for killing bacteria in medical operating rooms. Many other applications also exist.
A traditional circuit for controlling a mercury vapor lamp includes an AC power source which drives a primary side of a ballast transformer. A secondary side of the transformer is coupled to the lamp. The lamp includes a gas-filled tube with electrodes at each end of the tube. The secondary side of the transformer applies a voltage between the electrodes which accelerates electrons in the tube from one electrode toward the other. The electrons collide with gas atoms to produce positive ions and additional electrons. Since the current applied to the gas discharge lamp is alternating, the electrodes reverse polarity each half cycle.
Since the collisions between the electrons and the gas atoms generate additional electrons, an increase in the arc current causes the impedance of the lamp to decrease. This characteristic is known as "negative resistance." The lamp is unstable and current between the electrodes must be limited to avoid damaging the lamp. As a result, a typical control circuit includes a current limiting inductance coupled in series with the lamp. The inductance can either be a physically separate inductor or "built-in" to the transformer as a leakage inductance.
When the lamp is first started, the lamp requires a very large striking voltage to initiate an arc to ionize the gas in the lamp. The electrodes of the lamp are cold and there are almost no free electrons in the tube. The impedance of the lamp is therefore very high. The voltage required to initiate the arc exceeds that required to sustain the arc. For example, the ignition voltage may be 1,000 volts while the operating voltage may be 550 volts.
Lamp intensity adjustments have been made by providing a bank of capacitors in series with the lamp which are connected to one another in parallel through a plurality of mercury relay switches. The mercury relay switches are required to switch the high voltages (e.g. 1,000 volts) applied across the capacitor bank. The user changes the state of the switches to change the capacitance and thus the lamp intensity. The capacitor bank has several disadvantages. The capacitor bank is very large, expensive and unreliable. Also, the capacitor bank has a capacitance with only a few selectable magnitudes. This allows only a very coarse power adjustment. The mercury relay switches are expensive and in disfavor due to the environmental hazard presented by the mercury contained in the switches.
Lamp intensity adjustments have also been made by the use of tap-switching relays on the secondary side of the ballast transformer. This arrangement is also unreliable due to the high voltages present at the secondary side of the circuit.